Coupling light between optical fibers and waveguides is an indispensable part of any on- and off-chip optical communication system. However, coupling light from an optical fiber into a waveguide, or coupling light from a waveguide into an optical fiber, presents a number of problems. For example, conventional techniques for coupling light from a. waveguide into an optical fiber include configuring the end of the waveguide to scatter the light carried by the waveguide and positioning a lens at or near the end the optical fiber to direct a portion of the scattered light in the optical fiber core. For multimode fibers, a lens, can be used to direct the maximal ray of the focused beam within the numerical aperture of the fiber core, in this way the light captured by the lens can be effective coupled into the fiber core. On the other hand, in order to couple light into a single mode fiber, the mode of the fiber core and the mode of the light captured by the lens must be closely matched.
One conventional technique to couple light between a single mode fiber and a waveguide is to use a grating coupler. Light coming from the waveguide propagates through the grating and is scattered in free space at some finite angle from normal. The collecting fiber therefore needs to be positioned at an angle from normal for efficient coupling. Normal input/output operation is possible but not very efficient doe to the coupling between left and right propagating waves in the grating region. Although positioning the fiber at an angle can be done, it often complicates the design and packaging of the chip.
Physicists and engineers continue to seek enhancements that reduce the cost and increase the efficiency of optical fiber-to-waveguide couplers.